Not applicable.
1. Field of Invention
This invention relates to structural members and more particularly, to structural members made with foam exhibiting different rigidity and/or flexibility.
2. Description of the Related Art
Structural members for composite structures are commonly used in various applications such as cargo containers, vehicles, and boats. In the past, structural members have been developed using a variety of different technologies. One prior art technique for forming structural members for boat hulls uses plywood as the core of a sandwich laminate. The plywood acts to augment the structural properties of the fabric layer skins of the boat. Advantageously, plywood provides a low-cost means for providing structural rigidity. One significant drawback to working with plywood, however, is that it tends to decay significantly, often within five to seven years. Another drawback is the limited malleability of plywood for use in curved structures having small radii.
Another prior art technique for providing structural rigidity to composite structures makes use of a high-density structural foam core rather than a plywood core. The high-density structural foam is commonly formed from cross-linked PVC or very high-density urethane. Structural members having high-density structural foam cores and reinforced fabric skins commonly maintain their structural integrity for up to ten years. High-density structural foam is considerably more expensive than plywood, however, and is not cost effective to use in many applications, including boats. In addition, high-density foam, like plywood, is not sufficiently malleable or suitable for structural members requiring a tight curve. A tightly curved member made of foam or plywood is likely to crack or exhibit other structural defects when the structural member is positioned for example within a boat hull. These effects are due to strains on the outer curvature or stresses on the inner curvature of the member. For example, a relatively rigid structural member is disclosed in U.S. Pat. No. 5,429,066 to Lewit et al. which is assigned to the assignee of the present invention and herein incorporated by reference. The structural member disclosed in the ""066 patent comprises a fabric layer having an outer woven fabric layer and an inner non-woven fabric layer with a single structural foam core attached. The non-woven fabric layer, such as a mat fiber layer, is attached to the reinforcing fabric layer. A structural foam is attached to the non-woven fabric layer on the side of the non-woven fabric layer opposite the reinforcing fabric by filling the interstices of the non-woven fabric layer. However, the Lewit ""066 structure suffers from the inability to sufficiently bend for tight curvatures without deforming.
U.S. Pat. No. 5,908,591 to Lewit el al. concerns a composite structure having a structure similar to Lewit ""066 . Significantly, however, the Lewit ""591 composite structure does not make use of a second reinforcing fabric layer. Instead, penetration of the structural foam is controlled so as to leave an outer portion of the fabric layer of the cured composite structure substantially free of cured resin.
Thus, a need exists for a structural member which when laminated within a composite structure provides strength and rigidity, but is sufficiently malleable prior to lamination to allow for tight curvatures.
In a first aspect of the present invention, a method of forming a structural member comprises providing a cavity formed at least in part from a fabric layer and inserting at least a first foam core and a second foam core within the cavity. The second foam core preferably has a relatively higher rigidity than the first foam core and penetrates the fabric layer.
In another aspect of the present invention, a conformable composite reinforcing member comprises a cavity formed at least in part from a fabric layer and at least a first foam core and at least a second foam core positioned within the cavity, wherein the second foam core has a relatively higher rigidity than the first foam core.
In yet another aspect of the present invention, a method of forming a composite structure such as a boat comprises the steps of providing a structural member, confirming the structural member to a tight curvature of the composite structure, and hardening the structural member once it has conformed to the tight curvature.